High-frequency modulating system



L A E N E s N A H w w HIGH-FREQUENCY MODULATING SYSTEM Original Fil edDec. 30.1942

Patented Sept. 27, 1949 HIGH-FREQUEN CY MODULATING SYS'JIEIH William W.Hansen and Russell H. Variam'Garden City, N. Y., assignors to The SperryCorporation, a corporation of Delaware 1 Original application December30, 1942, Serial Divided and this application March 31, 1945, Serial No.586,007

18 Claims. 1

This invention relates, generally, to the control of ultra highfrequency electron beam velocity modulation tubes of the type disclosedin Fig. 2

of our U. S. Patent No. 2,250,511, entitled "Oscillator stabilizationsystem," issued July 29, 1941, and, more specifically, to novelmodulation means by which pure amplitude or frequency modulation can beobtained using such velocity modulation tubes.

The present application is a division of our copending applicationSerial No. 470,678 for High frequency modulating system, filed December30, 1942, which itself is a continuation-in-part of our copendingapplication Serial No. 420,770, now Patent No. 2,439,387, entitledElectronic tuning control," filed November 28, 1941.

As is shown in our U. S. Patent No. 2,281,935, entitled Modulationsystem, issued May 5, 1942, plural-resonator electron beam velocitymodulation tubes may have theircarrier outputs modulated by signalfrequencies. The most generally employed methods of modulation have beeneither to insert the signal voltage in series with the beam accelerationvoltage or to supply the signal voltage to a fine wire mesh control gridinterposed between the cathode and the entrance grid of the firstresonator'of the tube. If the action takes place in an oscillator tube,frequency as well as amplitude modulation will result.

One cause of frequency modulation depends on the well known fact that aspace filled with electrons has a dielectric constant which differs fromthe dielectric constant of free space. A variation of the electron beamcurrent by a signal voltage on the control grid alters the'dielectricconstant of the resonators through which the beam passes, thus changingthe effective tuning capacity and introducing a frequency change. Asecond source of frequency modulation lies in the change of phasebetween the driving voltage and the feedback voltage when the beamvelocity is altered by a signal voltage in series with the beamacceleration voltage. The condition that oscillation occurs when thephase angle around the complete regenerative circuit is equal to someintegral number times 21r radians indicates that any change in thetransit time in the bunching space due to changes in accelerationvoltage must be accompanied by a change in the phase angle betweenbuncher and "catcher resonators. This implies a frequency changeaccompanying the change in acceleration voltage. It is to be understoodthat frequency modulation will not occur in a modulated amplifier whichis isolated from the oscillator by a bufier resonator. This 55 densitywithin a common resonator substantially procedure is, however, oftenundesirable since three more resonators are added which complicate theapparatus and greatly increase the tuning problem.

On the other hand, under some circumstances, due to the sharpness of theresonance curves of the resonators themselves, it is difficult toproduce a pure frequency modulated carrier without simultaneousamplitude modulation.

It has been disclosed in above-mentioned Patent No. 2,281,935 that pureamplitude modula tion or pure frequency modulation may be obtained byusing combinations of acceleration voltage modulation and beam currentmodulation of the proper amount and in the correct phase relationship.This has the disadvantage, however, that.acceleration voltage modulationrequires a large signal amplitude and abstracts power from the signalsource.

In reflex type velocity modulation tubes, such as shown in Fig. 2 of ourPatent No. 2,250,511, similar modulation can be effected by variation ofthe beam-accelerating voltage or of the control grid voltage by themodulating signal.

According to the present invention, an improved system and method isprovided for obtaining, as desired, either amplitude or frequencymodulation in ultra high frequency oscillators of the reflex velocitymodulating type without the simultaneous occurrence of the undesiredform of modulation. Also according to the present invention a novelmethod is provided for obtaining predetermined relative amounts ofamplitude and frequency modulation in ultrahigh-fre- 4 quencyoscillators.

Another object of the present invention is to provide means forobtaining pure amplitude or pure frequency modulation with moderateamplitude modulating signals and negligible signal power consumption byimpressing the signal upon the control grid of a reflex velocitymodulation device and simultaneously upon a control grid coacting withan independent electron stream coacting with the electromagnetic fieldof said device.

Still another object is to modify the modulation characteristics of areflex velocity modulation tube by altering the dielectric constantwithin the resonator of the device with an auxiliary electron beam.

A further object of the present invention lies in the provision, in anultra frequency reflex velocity modulation tube, of an auxiliaryelectron beam adapted to maintai the average electron 3 constant whenthe average electron density of the main electron beam is modulated by asignal voltage.

Another object is to provide means and methods in an ultra, highfrequency tube of the character described for selectively obtaining.either pure amplitude modulation or pure frequency modulation by asimple operational adjustment.

Yet a further object is to provide, in an electron beam velocitymodulating oscillator having a cavity resonator, means for controllingthe average electron density within the resonator in accordance with asignal wave either to produce or to compensate for changes in theresonant frequency of the oscillator without imparting energy to thehigh frequency fieldtherein comprising means for projecting an electronbeam through the high frequency field of the resonator, and grid meansfor controlling the beam current, the beam being so directed as to becollected by the side of the resonator or other collector means withoutproviding feedback coupling with the field.

Other objects and advantages will become apparent from thespecification, taken in connection with the accompanying drawingswherein the invention is embodied in concrete form.

In the drawings,

Fig. 1 is a schematic wiring diagram of one form of our invention.

Figs. 2 and 3 are schematic wiring diagrams of modifications of theinvention.

similar characters of reference are used in all of the above figures toindicate corresponding parts.

It has been demonstrated mathematically and verified experimentally thatthe presence of free electrons within an electric field may eitherincrease or decrease the dielectric constant of the space containingthese charges from the unity value of a pure vacuum. For example, it isobserved in the study of the ionosphere that the dielectric constant maybe reduced to zero and even reversed in sign by the action of theelectrons. The effect is mathematically very complex but it is afunction, among others, of the electron density, the frequency of theelectric field, and the transit time of the electrons passing throughthe field. In general, it may be said that if the electrons areprojected in a low ve-- locity beam and remain within the electric fieldfor a duration long compared to a cycle of the field frequency, thedielectric constant is decreased by their presence. Contrarily,electrons projected through the electric field at a high velocity in atime short compared to a cycle of the field frequency cause an increasein the dielectrio constant. The ionosphere exhibits the former effectwhile most electron velocity modulating devices exhibit the lattereffect since electrons remain in the impressed electric fields for longand short durations, respectively. In the present invention both modesof operation are contemplated, and therefore, operating conditionsstated for explanatory purposes are not to be considered in any way tolimit the device to the exclusive employment of either high or lowvelocity electron beams.

Referring now to Fig. 1, there is seen diagrammatically a type of reflexelectron beam velocity modulating tube of the type shown in U. S. PatentNo. 2,250,511 but having a transverse electron control beam similar tothat disclosed in Fig. 3 of the above mentioned copending applicationSerial No. 420,770. The device comprises an elec- 4 tron emisslvecathode I35 from which an electron beam is accelerated by the potentialfrom a battery :36 through a mesh control grid I31 and thence through aresonator I having entrance and exit grids I39 and I, respectively.Behind exit grid MI is mounted a reflector plate 142 su plied with avoltage from a tap on the battery I36. The electron beam is reflectedback into the resonator I38 where it maintains an ultra high frequencyfield in the well known manner. A coaxial line I43 attached to acoupling loop I44 is adapted to remove energy from the resonatorAttached to the side of resonator I36 is a means' for producing anelectron beam transverse to the electric field of the resonator. Acathode I46 of this device produces an electron beam which isaccelerated through a mesh control grid I41 and through a grid I48 inthe side wall of resonator I38 by a voltage supplied by a battery I49,so that the beam passes substantially at right angles to the electronbeam from cathode I35 and between resonator grids I39, I41, preferablywithout impingin on these latter grids or the resonator walls supportingthese grids, being therefore collected on the opposite outer resonatorwall. Loop I44 and line I43 are shown in the plane of the paper forconvenience in drawing, but would preferably be located so that the beamfrom cathode I46 would not strike them. The modulation control grids I31and I41 are connected to the taps III and H2, respectively, of thepotentiom'eters H0 and I09, respectively, both fed from the modulatingsignal source I01 through transformer I06.

In operation, the beam from cathode I46 contributes no energy to aid inmaintaining the oscillating electromagnetic field within the resonator;in fact, it unavoidably extracts a small amount of energy. If the beamcurrent intensity from cathode I35 is increased, there results anincrease in the amplitude of oscillation of the electromagnetic fieldand a decrease in its frequency as a consequence of the tuning effect onthe resonator. An increase in the beam current from cathode I46,however, produces a decrease in output, due to its effect in loading theresonator, as well as a decrease or increase in output frequencydependent upon whether the control beam has a positive or negativeeffect on the dielectric constant of the resonator. Predeterminedamounts of amplitude and frequency modulation may be obtained,therefore, by the amplitude and phase adjustment of the signal voltageson the control grids I31 and I41.

Fig. 2 illustrates an alternate form of the present invention wherein areflex type of ultra high frequency oscillator has an auxiliary electronbeam projected at preferably a low velocity parallel to the excitingelectron beam. A resonator I5I of the device of Fig. 2 has across-section similar to that shown in Fig. 4 of parent application No.470,678. Cathode I52 provides an electron beam whose intensity iscontrolled by a grid I 53 and which is accelerated by a battery I54through entrance and exit grids 156 in the resonator I51. A reflectorelectrode or plate I55 maintained at an appropriate voltage by a tap onvoltage divider III connected across the battery I54 returns the beam tothe resonator I5Iwvhich is excited thereby in the usual manner.

Cathode I56 emits electrons which are projected by the accelerationvoltage of a battery I6I past control grid I51, and through asecond pairof entrance and exit grids I 59 in the resonator Q shown in Fig. 2wherein the resonator ISI is sep-- arated into a driving resonator INand a tuning resonator l5l" coupled by a coaxial line I81. Resonatorl5l", which may be similar to the resonators discussed with reference toFig. 11, of patent application No. 470,678, couples an impedance toresonator ISI' in a manner analogous to simple coupled circuits. Theoperation of the device shown in Fig. 3 is similar to that of Fig. 2 andconsequently needs no special explanation.

It appears obvious that the teachings of the present invention may beextended to other types of electron beam velocity modulation devicesthat have radial shooting exciting beams, annular resonators, andsimilar structural modifications. The usual tuning arrangements may beemployed with the illustrated embodiments of the present invention.Great latitude may be exercised in design of resonators, use ofsmoothing grids, em- 1 ployment of collector electrodes for mixeroperation, and'in similar engineering decisions.

As many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained inthe above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of simultaneously producing desired relative amounts ofamplitude and frequency modulation ranging from substantially pureamplitude modulation to substantially pure frequency modulation,comprising the steps of projecting a plurality of electron beams throughan electromagnetic field, reflecting one of said beams to re-passthrough said field to sustain oscillations thereof, collecting the otherof said beams after the first passage thereof through said field, andvarying the intensities of said beams in accordance with the same signalwaves with an amplitude and phase relationship dependent upon thedesired relative amounts of amplitude and frequency modulation of saidfield.

2. A method of simultaneously producing desired relative amounts ofamplitude and frequency modulation ranging from pure amplitudemodulation to substantially pure frequency modulation, comprising thesteps of projecting an electron beam through an ultra high frequencyelectromagnetic field, reversing and re-projecting said beam throughsaid field to sustain oscillation thereof, projecting an auxiliaryelectron beam solely for tuning purposes perpendicularly through atleast a portion of said field, modulating the intensities of saidelectron beams in accordance with signal waves having a predeterminedamplitude and phase relationship, and extracting energy from said fieldhaving the desired relative amounts .of amplitude and fresired relativeamounts'of amplitude and frequency modulation ranging from substantiallypure amplitude modulation to substantially pure frequency modulation,comprising the steps of maintaining an electromagnetic field inoscillation at a predetermined frequency by double traverse of anelectron beam therethrough, producing' an auxiliary unidirectionalelectron beam coupled to said field, and controlling both said electronbeams in accordance with the same signal waves with amplitude and phaserelationships dependent upon the desired relative amounts of amplitudeand frequency modulation of said field. v

4. An electron beam velocity-modulating device adapted to producesimultaneously desired relative amounts of amplitude and frequencymodulation, comprising cavity resonator means, means for projecting anelectron beam through said resonator means, means for reversing andre-projecting said beam through said resonator means to exciteelectromagnetic oscillations at a predetermined frequency therewithin,means for projecting an auxiliary electron beam through said resonatormeans to coact with said oscillations without imparting energy thereto,means for collecting said auxiliary beam after its passage through saidresonator means, and means for varying the intensities of said beams inaccordance with a signal wave, said last-mentioned means being adaptedto control the relative amplitudes and phases of said electron beamintensities dependent upon the desired relative amounts of amplitude andfrequency modulation.

5. Means for simultaneously producing desired relative amounts ofamplitude and frequency modulation ranging from substantially pure amplitude modulation to substantially pure frequency modulation,comprising resonator means. means for projecting an electron beamthrough said resonator means, means for reversing and re-projecting saidbeam through said resonator meansto excite an electromagnetic fieldtherewithin, means for projecting an auxiliary electron beam through atleast a portion of said field without imparting energy thereto, andmeans for simultaneously varying the intensities of said electron beamsin accordance with a signal wave and with an amplitude and phaserelationship dependent upon the desired relative amounts of amplitudeand frequency modulation.

6. In an electron beam velocity-modulating device having means forproducing an electron beam and a single resonator adapted both tovelocity-modulate said electron beam and to be electromagneticallyexcited by the. same beam upon subsequent reflection of the .latter indensity-modulated form through said resonator, the combinationcomprising means for varying the intensity of said exciting electronbeam in accordance with a modulating signal, means for projecting anauxiliaryelectron beam through said resonator, means for collecting saidauxiliary beam after initial passage thereof through said resonator, andmeans for varying the intensity of said auxiliary electron beam inaccordance with said modulating signal to maintain the average electrondensity in'said resonator substantially constant. v

7. Modulation control apparatus comprising a hollow cavity resonator,means for projecting an electron beam therethrough, means including areflector electrode adjacent said resonator for reversing said beam toreenter said resonator and to produce an electromagnetic fieldtherewithin,

enemas a second resonator coupled to said first resonator, means forprojecting an auxiliary electron beam through said second resonator,means for collecting said auxiliary beam after its initial passagethrough said second'resonator, and means for controlling said auxiliarybeam in accordance with a modulation signal to produce modulation ofsaid field.

8. Modulation control apparatus comprising means for producing anelectromagnetic field including a hollow cavity resonator, means forpro- J'ecting an electron stream therethrough, and means for reversingsaid stream to reenter said resonator; means for producing a.unidirectionally flowing auxiliary electron stream coupled to saidfield, and means for controlling both of said streams in accordance witha modulation signal to produce modulation of said field.

9-. Modulation control apparatus comprising means for producing anelectromagnetic field including a hollow cavity resonator, means forprojecting an electron stream therethrough, and means for reversing saidstream to reenter said resonator; means for producing a unidirectionallyfiowing auxiliary electron stream coupled to said field, means forcontrolling said first electron stream in accordance with a modulationsignal to produce modulation of said field, and means for modifying saidmodulation, comprising means for controlling said auxiliary electronstream by said same modulation signal.

10. High frequency apparatus comprising a hollow cavity resonator havingelectron-permeable walls, means for projecting a beam of electronsthrough said resonator by way of said walls, means for reversing saidelectrons to cause them to reenter said resonator to sustainoscillations therein, means for modulating said oscillations comprisingmeans for controlling said electron stream by a modulation signal, andmeans for controlling the modulation of said output comprising a secondcavity resonator coupled to said first resonator, means for passing astream of electrons through said second resonator, and means forcontrolling said second stream by said signal.

11. Modulation control apparatus for simultaneously producingpredetermined amounts of amplitude and frequency modulation, comprisingfirst and second discharge devices each having an electron gun, amodulation control grid, 2. resonator, and a electrode, located in axialconsecution,

said electrode of one of said devices being connected to a source ofground potential, a modulator having a single signal source andconnected to said modulation control grids of said devices, an energycoupler directly connecting the resonators of said devices, and meansfor extracting energy from one of said resonators.

12. Apparatus for simultaneously producing predetermined relativeamounts of amplitude and frequency modulation ranging from substantiallypure amplitude modulation to substantially pure frequency modulation,comprising a first velocity modulation discharge device having anelectron gun, modulation control grid, resonator, and a reflector, alllocated in axial consecution, said modulation control grid beingconnected to a source of predetermined direct potential, a seconddischarge device having an electron gun,

a modulation control grid, and an electrode, all

tion signal source connected to said modulation control grids and saiddevices, and an energy coupler connecting the resonators of saiddevices.

13. Modulation control app ratus c prisin a source of first and secondbeams of electrons. an electron resonant chamber means having -apredetermined resonant frequency, a modulator connected to receive boththe first and second beams of electrons and having a single source ofmodulation signal, means for defining a first beam path through saidresonant chamber means for receiving said first beam of electrons, saidfirst beam path having a transit time length substantially long comparedto the duration of a cycle of said predetermined resonant frequency,further means for defining a second beam path through said resonantchamber means for receiving said second beam 0f electrons, said secondbeam path having a transit time length substantially short compared tosaid transit time length of said first beam path, and means forextracting energy from said resonant chamber means, said energy havingpredetermined relative amounts of amplitude and frequency modulation.

14. Modulation control apparatus, comprising a first and second sourceof electron beams, a first and second modulation means connected toreceive said first and second electron beams respectively,electromagnetic resonant chamber means having a predetermined resonantfrequency and further having two reentrant .portlons each defininganelectron permeable gap, means for directing said first and secondbeams of electrons through said electron permeable gaps, a first andsecond reflector plate connected in axial alignment withthe paths ofsaid first and second beams of electrons respectively, one of saidreflector plates being connected .to a substantially higher potentialthan the other said reflector plate, and energy extracting meansconnected to said resonator chamber means.

15. A method of simultaneously producing desired relative amounts ofamplitude and frequency modulation, comprising the steps of maintainingan electromagnetic field in oscillation at a predetermined frequency bypassing an electron beam therethrough in a time duration substantiallyshort compared with the period of said predetermined frequency, anddirecting a second beam through said field in a time durationsubstantially long compared with said predetermined frequency, one ofsaid beams being directed in a double traverse of said electromagneticfield and the other of said beams in a single traverse of said field.

16. A method of simultaneously producing desired relative amounts ofamplitude and frequency modulation ranging from substantially pureamplitude modulation to substantially pure frequency modulation,comprising the steps of maintaining an electric field in oscillation atpredetermined frequency by double traverse of a first electron :beamtherethrough in a time duration substantially short compared with theperiod of said predetermined frequency, and directing a second auxiliarybeam through said field in a time duration substantially long comparedwith said predetermined frequency.

17. A method of simultaneously producing desired relative amounts ofamplitude and frequency modulation ranging from substantially pureamplitude modulation to substantially pure frequency modulation,comprising the steps of maintaining an electric field in oscillation atpredetermined frequency by double traverse of a first electron beamtherethrough in a time duration substantially short compared with theperiod of said predetermined frequency, directing a sec ond auxiliarybeam through said field in a time duration substantially long comparedwith said predetermined frequency, and modulating both said beams withthe same modulating signal.

18. A method of eliminating unwanted frequency modulation in a velocitymodulation discharge device having a cathode, a modulating grid, aresonator, and a reflector plate, comprising the steps of directing anauxiliary beam of electrons through a portion of the electromagneticfield of said resonator in a time duration substantially long comparedwith the frequency of said resonator, and modulating said auxiliary beamwith the same modulation signal as said modulating grid.

WILLIAM W. HANSEN. RUSSELL H. VARIAN.

10 REFERENCES crrzn The following references are of record in the fileof this patent:

UNITED STATES PATENTS

